U.S. researchers in Africa said that they found that circumcision is such a good defense against HIV infection that they shut down two studies early, and instead offered all participants a chance to be circumcised.

One study in the east African country of Kenya showed that circumcision cut adult males' HIV infection risk from heterosexual intercourse by 53 percent, while another study in Uganda lowered the risk by 48 percent, according to results released.

The findings, financed by the U.S. National Institutes of Health (NIH), pointed out that the latest conclusions confirmed previous investigations into the value of circumcision as a protection against HIV, the virus that causes AIDS. This is especially important in Africa, where AIDS is an epidemic in many countries, infecting an estimated 25 million people on the continent.

Despite the good news, there is still plenty of reason for caution, AIDS experts said.

"Male circumcision is a difficult intervention to implement, and the preventive effect is relative, not absolute," said Thomas Coates, an AIDS specialist and a professor of medicine at the University of California at Los Angeles. "The magnitude of effect is 50 to 60 percent, which still leaves ample room for people to get infected with HIV."

There are other caveats as well: The study did not look at male-to-female transmission, and it was also not clear whether circumcision makes it less likely that gay men could transmit HIV to each other.

In the United States, homosexual transmission of HIV is more common than heterosexual transmission, the experts said. And most men in the United States are circumcised, making the procedure less effective as a possible prevention tool.

Still, the findings could have plenty of meaning in Africa, where HIV is commonly spread between men and women.

Studies have suggested the value of circumcision in the past, but researchers wanted to confirm the previous findings.

According to the NIH, most adult Africans are circumcised, but the rate drops below 20 percent in some areas of southern Africa where HIV and AIDS are common.

In one of the two studies, researchers enrolled 2,784 HIV-negative, uncircumcised men in Kenya beginning in 2002. The other study, in Uganda, started in 2003 and enrolled 4,996 HIV-negative, uncircumcised men.

Some of the men were assigned to immediately undergo circumcision, while others had to wait two years.

Then researchers studied whether the circumcision had any effect on their rates of getting HIV.

The results were so encouraging that an oversight board halted the studies this week, and ordered that all participants be given circumcisions instead of having to wait.

In Kenya, researchers found that only 22 of the 1,393 circumcised men in the study were infected with HIV, compared to 47 of the 1,391 men who had yet to be circumcised.

The numbers for Uganda weren't immediately available.

"Circumcision is now a proven, effective prevention strategy to reduce HIV infections in men," Robert Bailey, a study investigator and professor of epidemiology at the University of Illinois at Chicago, said in a statement.

It's not entirely clear how circumcision reduces HIV infection. But researchers have suggested that the foreskin may provide a moist, safe environment for the AIDS virus and provide more immune cells for HIV to infect.

Coates called the study results the "second greatest finding in HIV prevention," right behind research that confirmed drugs could stop mother-to-baby transmission of the AIDS virus.

Still, he added, "combination prevention" remains crucial -- combining circumcision with using condoms, reducing sexual partners, and delaying the first time people have intercourse.

The Associated Press reported that the link between male circumcision and HIV prevention was first noted in the late 1980s. The first major clinical trial, of 3,000 men in South Africa, found last year that circumcision cut the HIV risk by 60 percent.